U.S. patent application number 11/437874 was filed with the patent office on 2007-01-04 for moisture resistant microphone.
This patent application is currently assigned to InSound Medical, Inc.. Invention is credited to Ian Michael Day, Richard Gable, Michael Ipsen, Sunder Ram.
Application Number | 20070003081 11/437874 |
Document ID | / |
Family ID | 37605145 |
Filed Date | 2007-01-04 |
United States Patent
Application |
20070003081 |
Kind Code |
A1 |
Ram; Sunder ; et
al. |
January 4, 2007 |
Moisture resistant microphone
Abstract
Embodiments of the invention provide microphone assemblies which
are resistant to moisture. One embodiment provides a microphone
assembly comprising a housing, a diaphragm disposed in the housing
and a backplate disposed in the housing. The housing includes a
sound inlet port for the entry of sound waves. The backplate
includes a surface and an electret portion having an embedded
permanent charge. The diaphragm is configured to vibrate in
response to sound waves entering the housing. The vibrations of the
diaphragm interact with the electret portion to produce an
electrical signal associated with the sound waves entering the
housing. A hydrophobic coating can be applied to one or both of the
backplate and diaphragm surfaces so as to reduce condensation
and/or wetting of the backplate. This minimizes neutralization of
an electric field of the backplate surface from condensation
preserving the field and the function of the microphone in humid
environments.
Inventors: |
Ram; Sunder; (San Jose,
CA) ; Gable; Richard; (Sunnyvale, CA) ; Ipsen;
Michael; (Redwood City, CA) ; Day; Ian Michael;
(Fremont, CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER
EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
InSound Medical, Inc.
Newark
CA
|
Family ID: |
37605145 |
Appl. No.: |
11/437874 |
Filed: |
May 19, 2006 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60696265 |
Jun 30, 2005 |
|
|
|
Current U.S.
Class: |
381/191 |
Current CPC
Class: |
H04R 1/086 20130101;
H04R 25/652 20130101; H04R 19/016 20130101; H04R 2225/023 20130101;
H04R 25/654 20130101; H04R 25/602 20130101; H04R 2410/00
20130101 |
Class at
Publication: |
381/191 |
International
Class: |
H04R 25/00 20060101
H04R025/00 |
Claims
1. A microphone assembly for a hearing aid, the assembly
comprising: a microphone housing sized for use with a hearing aid,
the housing including a sound inlet port for the entry of sound
waves into the housing; a diaphragm disposed in the housing, the
diaphragm configured to vibrate in response to sound waves entering
the housing; a backplate disposed in the housing at an offset from
a surface of the diaphragm, the backplate including an electret
portion having an embedded permanent charge, wherein vibrations of
the diaphragm electrically interact with the electret portion to
produce an electrical signal associated with the sound waves
entering the housing; and a hydrophobic coating disposed on the
diaphragm for reducing water condensation to minimize
neutralization of an electric field strength of the backplate
surface, a coating thickness configured to have minimal effect on
acoustical vibrations of the diaphragm.
2. The microphone assembly of claim 1, wherein the coating has
minimal effect on at least one of an amplitude of the acoustical
vibrations of the diaphragm or a resonant frequency of the
diaphragm.
3. The microphone assembly of claim 1, wherein the hydrophobic
coating is disposed on a backplate surface facing the
diaphragm.
4. The microphone assembly of claim 1, wherein the hydrophobic
coating is disposed on at least a portion of a microphone housing
interior surface.
5. The microphone assembly of claim 1, wherein the hydrophobic
coating is disposed on at least a portion of a microphone housing
exterior surface.
6. The microphone assembly of claim 1, wherein the hearing aid is a
CIC hearing aid.
7. The microphone assembly of claim 1, wherein the electret
comprises at least one of a fluoropolymer, polytetrafluoroethylene,
TEFLON or polycarbonate.
8. The microphone assembly of claim 1, wherein the coating
comprises a fluoropolymer.
9. The microphone assembly of claim 1, wherein the coating has a
surface energy in the range of about 11 to 12 dynes/cm.
10. The microphone assembly of claim 1, wherein the coating
comprises a room temperature curable polymer.
11. The microphone assembly of claim 1, wherein the coating has a
thickness of about 1 micron.
12. The microphone assembly of claim 1, wherein the coating has a
volume resistivity of about 4.6.times.10.sup.12 ohm cm.
13. The microphone assembly of claim 1, wherein the diaphragm
comprises at least one of a polymer, a polyurethane or a polymer
with a conductive coating.
14. The microphone assembly of claim 1, further comprising an
integrated circuit electrically coupled to the diaphragm for
processing the electrical signals.
15. The microphone assembly of claim 14, wherein the integrated
circuit includes a pre-amplification circuit.
16. The microphone assembly of claim 14, wherein the integrated
circuit is positioned on a side of the diaphragm opposite to that
of the backplate.
17. The microphone assembly of claim 14, wherein the coating is
applied to a surface of the diaphragm adjacent the integrated
circuit so as to protect the integrated circuit from condensation
from the diaphragm.
18. A hearing aid comprising: the microphone assembly of claim 1; a
receiver assembly for converting the electrical signal into an
acoustical output; and a power source.
19. The hearing aid of the microphone of claim 18, wherein the
hearing aid is a CIC hearing aid.
20. The hearing aid of claim 19, wherein the coating is configured
to minimize wetting of the backplate surface when the hearing aid
is used for periods of extended continuous wear in the ear
canal.
21. The hearing aid of claim 20, wherein the period is up to six
months.
22. A method for using a hearing aid, the method comprising,
inserting the hearing aid of claim 18 into the ear canal of a
wearer; and wearing the hearing aid in the ear canal.
23. The method of claim 18, wherein the hearing aid is worn in a
bony portion of the ear canal.
24. The method of claim 22, wherein the hearing aid is exposed to
changes in ambient temperatures without appreciable degradation in
microphone performance.
25. The method of claim 22, wherein the hearing aid is exposed to
high humidity conditions without appreciable degradation in
microphone performance.
26. The method of claim 22, wherein microphone performance does not
appreciably degrade when a temperature of a portion of the
microphone assembly becomes less than a dew point temperature for
the environment in the canal.
27. The method of claim 22, wherein the hearing aid is worn for an
extended period without appreciable degradation in microphone
performance.
28. The method of claim 27, wherein the extended period is up to
six months.
29. A method for improving the resistance of a hearing aid
microphone to condensation, the method comprising: coating at least
one of a backplate or diaphragm of a hearing aid microphone with a
hydrophobic coating configured to reduce liquid condensation on the
backplate, the coating configured to have minimal effect on
acoustical vibrations of the diaphragm; and assembling the
microphone into a hearing aid.
30. The method of claim 29, wherein the hearing aid is a CIC
hearing aid.
31. The method of claim 29, wherein the coating is applied by dip
coating the microphone in a coating solution.
32. The method of claim 29, further comprising: substantially
maintaining a charge on the backplate surface when the microphone
is exposed to high humidity conditions.
33. The method of claim 29, further comprising: substantially
maintaining a charge on the backplate surface when the microphone
is exposed to changes in ambient temperature.
34. The method of claim 29, further comprising: substantially
maintaining an output signal from the microphone when the
microphone is exposed to changes in ambient temperature.
35. The method of claim 29, further comprising: substantially
maintaining an output signal from the microphone when a portion of
the diaphragm or the backplate is at or below a dewpoint
temperature for the environment in the ear canal.
36. A microphone assembly for a hearing aid, the assembly
comprising: a microphone housing sized for use with a hearing aid,
the housing including a sound inlet port for the entry of sound
waves into the housing; a diaphragm disposed in the housing, the
diaphragm configured to vibrate in response to sound waves entering
the housing; a backplate disposed in the housing at an offset from
a surface of the diaphragm, the backplate including a surface
facing the diaphragm and an electret portion having an embedded
permanent charge, wherein vibrations of the diaphragm electrically
interact with the electret portion to produce an electrical signal
associated with the sound waves entering the housing; and a
hydrophobic coating disposed on the backplate surface for reducing
water condensation on the backplate surface so as to minimize
neutralization of an electric field strength of the backplate
surface, a coating thickness configured to have minimal effect on
electrical interactions of the diaphragm with the backplate.
37. The microphone assembly of claim 36, wherein the coating
comprises a fluoropolymer.
38. The microphone assembly of claim 36, wherein the coating has a
surface energy in the range of about 11 to 12 dynes/cm.
39. The microphone assembly of claim 36, wherein the coating
comprises a room temperature curable polymer.
40. The microphone assembly of claim 36, wherein the coating has a
thickness of about 1 micron.
41. A microphone assembly for a hearing aid, the assembly
comprising: a microphone housing sized for use with a hearing aid,
the housing including a sound inlet port for the entry of sound
waves into the housing; a diaphragm disposed in the housing, the
diaphragm configured to vibrate in response to sound waves entering
the housing, a backplate disposed in the housing at an offset from
a surface of the diaphragm, the backplate including an electret
portion having an embedded permanent charge, wherein vibrations of
the diaphragm electrically interact with the electret portion to
produce an electrical signal associated with the sound waves
entering the housing; and a hydrophobic coating disposed on the
diaphragm for reducing water condensation to minimize
neutralization of an electric field strength of the backplate
surface, wherein the coated diaphragm is acoustically operable
through a range of audible sound to provide an electrical signal
usable by the hearing aid to provide an acoustical output that is a
discernable representation of an audible sound input.
42. The microphone assembly of claim 41, wherein the coating
comprises a fluoropolymer.
43. The microphone assembly of claim 41, wherein the coating has a
surface energy in the range of about 11 to 12 dynes/cm.
44. The microphone assembly of claim 41, wherein the coating
comprises a room temperature curable polymer.
45. The microphone assembly of claim 41, wherein the coating has a
thickness of about 1 micron.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority of U.S.
Provisional Patent Application Ser. No.: 60/696265, entitled,
Hearing Aid Microphone Protective Barrier filed on Jun. 30, 2005,
the full disclosure of which is herein incorporated by
reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] Embodiments of invention relate to protective coatings for
microphones. More specifically, embodiments of the invention relate
to moisture protective coating for components used in microphones
such as condenser microphones. Still more specifically, embodiments
relate to moisture protective coating for microphone components
used in hearing aids such as completely in the canal hearing
aids.
[0003] Since many hearing aid devices are adapted to be fit into
the ear canal, a brief description of the anatomy of the ear canal
will now be presented. While, the shape and structure, or
morphology, of the ear canal can vary from person to person,
certain characteristics are common to all individuals. Referring
now to FIGS. 1-2, the external acoustic meatus (ear canal) is
generally narrow and contoured as shown in the coronal view in FIG.
1. The ear canal 10 is approximately 25 mm in length from the canal
aperture 17 to the center of the tympanic membrane 18 (eardrum).
The lateral part (away from the tympanic membrane) of the ear
canal, a cartilaginous region 11, is relatively soft due to the
underlying cartilaginous tissue. The cartilaginous region 11 of the
ear canal 10 deforms and moves in response to the mandibular (jaw)
motions, which occur during talking, yawning, eating, etc. The
medial (towards the tympanic membrane) part, a bony region 13
proximal to the tympanic membrane, is rigid due to the underlying
bony tissue. The skin 14 in the bony region 13 is thin (relative to
the skin 16 in the cartilaginous region) and is more sensitive to
touch or pressure. There is a characteristic bend 15 that roughly
occurs at the bony-cartilaginous junction 19 (referred to herein as
the bony junction), which separates the cartilaginous 11 and the
bony 13 regions. The magnitude of this bend varies among
individuals.
[0004] A cross-sectional view of the typical ear canal 10 (FIG. 2)
reveals generally an oval shape and pointed inferiorly (lower
side). The long diameter (D.sub.L) is along the vertical axis and
the short diameter (D.sub.S) is along the horizontal axis. These
dimensions vary among individuals.
[0005] Hair 5 and debris 4 in the ear canal are primarily present
in the cartilaginous region 11. Physiologic debris includes cerumen
(earwax), sweat, decayed hair and skin, and oils produced by the
various glands underneath the skin in the cartilaginous region.
Non-physiologic debris consists primarily of environmental
particles that enter the ear canal. Canal debris is naturally
extruded to the outside of the ear by the process of lateral
epithelial cell migration (see e.g., Ballachanda, The Human ear
Canal, Singular Publishing, 1995, pp. 195). There is no cerumen
production or hair in the bony part of the ear canal.
[0006] The ear canal 10 terminates medially with the tympanic
membrane 18. Laterally and external to the ear canal is the concha
cavity 2 and the auricle 3, both also cartilaginous. The junction
between the concha cavity 2 and the cartilaginous part 11 of the
ear canal at the aperture 17 is also defined by a characteristic
bend 12 known as the first bend of the ear canal.
[0007] First generation hearing devices were primarily of the
Behind-The-Ear (BTE) type. However they have been largely replaced
by In-The-Canal hearing devices are of which there are three types.
In-The-Ear (ITE) devices rest primarily in the concha of the ear
and have the disadvantages of being fairly conspicuous to a
bystander and relatively bulky to wear. Smaller In-The-Canal (ITC)
devices fit partially in the concha and partially in the ear canal
and are less visible but still leave a substantial portion of the
hearing device exposed. Recently, Completely-In-The-Canal (CIC)
hearing devices have come into greater use. These devices fit deep
within the ear canal and can be essentially hidden from view from
the outside.
[0008] In addition to the obvious cosmetic advantages, CIC hearing
devices provide, they also have several performance advantages that
larger, externally mounted devices do not offer. Placing the
hearing device deep within the ear canal and proximate to the
tympanic membrane (ear drum) improves the frequency response of the
device, reduces distortion due to jaw extrusion, reduces the
occurrence of the occlusion effect and improves overall sound
fidelity.
[0009] However despite their advantages, many completely CIC
hearing devices have performance and reliability issues relating to
the exposure of their components to liquid water such as that from
condensation, perspiration or water entering the ear canal. The
hearing aid microphone assembly (which typically includes a housing
having a sound port and an internal components) can be particular
susceptible when water enters the microphone housing through the
sound port and compromises the performance of the internal
components. Some current hearing aids use a type of variable
capacitor (or condenser) microphone known as an electret microphone
which translates acoustical energy into electrical signals through
the use of a backplate having an embedded charged forming one plate
of the capacitor and a movable diaphragm forming the other.
Movement of the diaphragm in response to sound waves alters the
capacitance of the microphone, in turn varying the voltage between
the backplate and the diaphragm resulting in the output signal from
the microphone. However, the presence of liquid water from
condensation or other sources can neutralize the electrical charge
of the backplate surface so as to attenuate the output signal of
the microphone or otherwise adversely affect the performance of the
microphone. Condenser microphones are particularly susceptible to
condensation when coming in from a cold environment to a warm
environment.
[0010] While porous protective covers can be employed to cover the
microphone sound port to prevent entry of liquid water into the
housing of the microphone such approaches still allow water vapor
to enter the microphone and condense onto internal surfaces of the
microphone. Also, such approaches don't protect the microphone
components if liquid water should enter the microphone by other
routes and they may impede acoustical conduction into the
microphone. Thus, there is a need for condenser microphone design
having improved resistance to water condensation and exposure to
other sources of liquid water. There is also a need for hearing aid
microphones having an improved resistance to water condensation
exposure to other sources of liquid when the hearing device is
positioned in the ear canal.
BRIEF SUMMARY OF THE INVENTION
[0011] Various embodiments of the invention provide devices,
assemblies and methods for improving the resistance of microphones
such as condenser microphones to moisture and condensation. Many
embodiments provide assemblies, devices and methods for improving
the resistance of hearing aid microphones to moisture and
condensation. Such hearing aid microphones can include those used
in CIC hearing aids including CIC hearing aids configured to be
worn in the bony portion of the ear canal for extended periods.
Particular embodiments provide hydrophobic-coated microphone
assemblies for improving the resistance of hearing aid microphones
to moisture and condensation. Such embodiments can include
hydrophobic-coated components for electret and other condenser
based microphones.
[0012] Particular embodiments provide a microphone assembly for a
hearing aid comprising a microphone housing, a diaphragm disposed
in the housing and a backplate disposed in the housing. The housing
includes a sound inlet port for the entry of sound waves into the
housing. The housing is sized for use with a hearing aid such as a
CIC hearing aid. The backplate includes a surface and an electret
portion having an embedded permanent charge. The diaphragm is
configured to vibrate in response to sound waves entering the
housing. The vibrations of the diaphragm interact with the electret
portion to produce an electrical signal associated with the sound
waves entering the housing. In many embodiments, an integrated
circuit can be electrically coupled to the diaphragm to process the
electrical signal. The integrated circuit can for example, convert
the change in capacitance into voltage or impedance it can also
perform pre-amplification of the signal for further processing.
[0013] A hydrophobic coating can be applied to one or both of the
backplate and the diaphragm surfaces so as to reduce wetting and/or
water condensation on the backplate. This in turn, minimizes the
neutralization of the electric field of the backplate surface from
such condensation and thus preserves and stabilizes the electric
field at the backplate surface. Typically, the coating will be
applied to the backplate surface facing the diaphragm and versa
visa. In one embodiment, the coating need only be applied to the
backplate surface facing the diaphragm. Also, the thickness of the
coating on either part is desirably configured to have minimal
effect on the acoustical vibrations of the diaphragm as well as the
electrical interactions of the diaphragm with the backplate. This
in turn, minimizes any effects on acoustical parameters of the
microphone such as microphone sensitivity, distortion level,
bandwidth or like parameter, or the interaction of the backplate
with the vibrating diaphragm. The coated membrane is thus
acoustically operable through the range of audible sounds to
provide an electrical signal usable by the hearing aid to provide
an acoustical output that is a discernable representation of an
audible sound inputted to the diaphragm. Desirably, the coating has
a surface energy equal or less than that of the backplate surface.
Also desirably, the thickness of the coating is small in relation
to the offset distance e.g., less than 10%, and more preferably
less than 5%. In preferred embodiments, the coating is a
fluoropolymer, has a thickness of about 1 um and a surface energy
(also described as surface tension) of about 11 to 12 dynes/cm but
other materials with other properties are equally applicable. Also
in preferred embodiments, the coating cures at room temperature and
the coating solution is substantially free of pigments or other
solids, which may absorb water or cause surface asperties. In a
particular embodiment, the coating is a fluoropolymer and is
applied to backplate having a fluoropolymer surface. The coating
can also be applied throughout the interior and the exterior of the
housing to not only reduce wetting and/or condensation on the
backplate, but also prevent moisture from wicking into the housing
through the sound port.
[0014] In another aspect of the invention, a hearing aid is
provided that includes an embodiment of the microphone assembly
described herein, a receiver assembly for converting the electrical
signal from the microphone into an acoustical output and a power
source for powering the hearing aid. The hearing aid can include a
CIC hearing aid configured to be worn continuously in the ear canal
for extended periods, e.g., six months or longer. Accordingly, the
coating can be configured to minimize condensation on the backplate
for periods of extended continuous wear of the hearing aid in the
ear canal. This can include periods of extended continuous wear
when the hearing aid is worn in the bony portion of the ear canal.
Other embodiments of the invention contemplate use of the coated
microphone in CIC hearing aids positioned in other portions of the
ear, as well as in ITE and BTE hearing aids. In still other
embodiments, the coating can be used for other types of microphones
including sound microphones for recording or amplification
including all-weather microphones configured for use indoors and
outdoors. The particular coating and coating properties can be
selected for a particular range of expected ambient conditions.
[0015] The coating can be configured to provide condensation
protection to the microphone assembly for high humidity conditions
within the ear canal including for relative humidities of 90% or
greater at temperatures approximating body temperature. The coating
can also be configured to not only reduce condensation in the humid
environment of the ear canal, but do so when the wearer rapidly
changes his ambient thermal environment such as coming from a cold
outside environment to a heated indoors. Specific embodiments can
be configured to provide condensation protection for sudden
temperature changes of 10 or 20.degree. C. or even greater. By
reducing or preventing condensation, the coating serves to maintain
a charge on the backplate surface and in turn maintain microphone
performance in cases of rapid fluctuations in ambient temperature.
The coating also improves long term reliability of the hearing aid
by reducing or preventing the collection of liquid water within the
microphone housing that may damage one or more electrical
components of the hearing aid. This and related embodiments thus
provide the wearer with an all-weather wear hearing aid allowing
the wearer to freely move from one thermal environment to another
without degradation in hearing aid performance both in the short
term and in the long term. This all weather capability is
particularly useful for extended wear hearing aids because the user
need not remove the hearing aid in various thermal environments and
the useful life of the hearing aid is extended allowing longer
periods of wear.
[0016] Other embodiments of the invention provide methods for
coating the microphone components such as the backplate and
diaphragm with the hydrophobic coating. Such methods can include
dip coating, spray coating and the like. Particular embodiments
provide methods and coatings in which the coating can be cured at
room temperature so as to minimize the potential for thermal damage
to various components of the microphone including the diaphragm,
backplate and various electrical components. Such embodiments
provide a method of coating which reduces the probability of
thermal failure of various microphone components. Further aspects
and embodiments of the invention are described in the detailed
description below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a side coronal view of the external ear canal;
[0018] FIG. 2 is a cross-sectional view of the ear canal in the
cartilaginous region;
[0019] FIG. 3 is a lateral view illustrating an embodiment of a
hearing aid device positioned in the bony portion of the ear
canal.
[0020] FIG. 4A is a cross-sectional view illustrating an embodiment
of the microphone assembly.
[0021] FIG. 4B is a block diagram illustrating the electrical
function of the microphone assembly.
[0022] FIG. 5 is a cross-sectional view illustrating the spacing
between the backplate and the diaphragm.
[0023] FIG. 6A is a cross-sectional view illustrating the placement
of the electret portion in the backplate
[0024] FIG. 6B is a cross-sectional view illustrating a coated
backplate with electret portion below the surface of the
backplate.
[0025] FIG. 6C is a cross-sectional view illustrating the coated
backplate with electret portion at the surface of the
backplate.
[0026] FIG. 7A is a cross-sectional view illustrating coating of
the backplate and one side of the diaphragm
[0027] FIG. 7B is a cross-sectional view illustrating coating of
the backplate and both sides of the diaphragm.
[0028] FIG. 7C is a cross-sectional view illustrating coating of
the backplate only.
DETAILED DESCRIPTION OF THE INVENTION
[0029] Various embodiments of the invention provide devices,
assemblies and methods for improving the resistance of microphones
such as condenser microphones to moisture and condensation. Many
embodiments provide assemblies, devices and methods for improving
the resistance of hearing aid microphones to moisture and
condensation. Such hearing aid microphones can include those used
in CIC hearing aids including those worn in the bony portion of the
ear canal for extended periods and in varying environmental
conditions. Particular embodiments provide hydrophobic-coated
microphone assemblies for improving the resistance of hearing aid
microphones to moisture and condensation including electret
microphones.
[0030] Referring now to FIG. 3, since many embodiments provide
moisture resistant microphone assemblies that can be used in CIC
hearing aids, an embodiment of a CIC hearing aid will now be
described. Though it should be appreciated, that this is but one
type of hearing aid that can utilize embodiments of the microphone
assembly described herein and other hearing aids are equally
applicable. CIC hearing aid 20 is configured for placement and use
in ear canal 10 and can include a receiver (speaker) assembly 25, a
microphone assembly 30, a battery assembly 26 and one or more
sealing retainers 27 coaxially positioned with respect to receiver
assembly 25 and/or microphone assembly 30. Receiver assembly 25 is
configured to supply acoustical signals received from the
microphone assembly to a tympanic membrane of the wearer of the
device. Preferably, device 20 is configured for placement and use
in the bony region 13 of canal 10 so as to minimize acoustical
occlusion effects due to residual volume 6 of air in the ear canal
between device 20 and tympanic membrane 18. Further description of
hearing aid 20 can be found in U.S. Pat. Nos. 6,473,513 and
6,940,988 and U.S. patent application Ser. No. 11/058,097 which all
are incorporated by reference herein in their entirety.
[0031] Also, since various embodiments of the invention relate to
microphones including condenser microphones, a background
discussion of microphones will now be presented. Condenser
microphones are one of the more commonly used microphones in many
acoustic application including hearing aids. These types of
microphones use a lightweight thin membrane (commonly referred to
as a diaphragm) and a fixed plate (commonly referred to as a
backplate) that act as opposite sides or plates of a capacitor. The
backplate is offset a set distance from the diaphragm. The
backplate and the diaphragm are charged with respected to one
another, typically through the use of a polarizing voltage. Sound
pressure against the diaphragm causes it to vibrate changing the
offset between the plates. The varying offset caused by the
vibrations changes the capacitance of the plates and in turn,
changes, the voltage between the plates. This changing voltage
comprises the output signal from the microphone.
[0032] Many hearing aids use type of condenser microphone known as
an electret microphone. In an electret microphone, the backplate
includes a type of dialectic material known as an electret which
has a permanently embedded charge analogous to a permanent magnet.
This charge eliminates the need to have an external bias voltage
between the plates. However as discussed above, when water droplets
condense on the surface of the backplate, the charge at the surface
of the backplate can be neutralized by charged particles in the
water so as to adversely affect the performance of the
microphone.
[0033] Referring now to FIGS. 4A-7C, an embodiment of a moisture
and condensation resistant microphone assembly 30 includes a
housing 40, and a backplate 50 and diaphragm 60 disposed within the
housing. Housing 40 includes an interior space 41, an interior
surface 42, an exterior surface 43 and a sound port 45 for entry of
sound waves 70 into the housing interior. The housing can be
fabricated from various rigid metals or polymers known in the art
such as polystyrene, HDPE, LDPE, and like materials. It can be
formed by one or more of molding, micro-machining, stereo
lithography and like methods. Also it will typically comprise two
or more portions (e.g., halves) which are joined together by
adhesive, ultrasonic welding, solvent bonding, or other joining
method known in the art. In one embodiment, one or more components
of the microphone assembly (e.g., the backplate) can be integral to
the housing and can be formed from the housing itself through
stereo lithography, micromachining and like methods. In embodiments
of the invention adapted for hearing aids, the microphone assembly
housing 40 is sized to fit into a hearing aid structure such as a
module or housing. Such hearing aid structures can include those
used in CIC hearing aids configured to be positioned in the bony
portion of the ear canal.
[0034] The backplate has a first side 51 (also described as surface
51) facing the diaphragm and a second side 52 (also described as
surface 52) facing away and an electret portion 80. Also, the
backplate typically has an opening 55 for the transmission of sound
to diaphragm 60. The opening can be substantially vertically
aligned with housing sound port 45 or it can be offset a selected
distance. The latter configuration provides a baffling effect to
minimize the likelihood of direct fluid migration from sound port
45 to the backplate. In various embodiments, the backplate can be
fabricated from one or more rigid polymers including various
polycarbonates and fluoropolymers. In preferred embodiments, at
least a portion of the backplate comprises PTFE, an example
including TEFLON available from the DuPont Nemours Corporation. In
embodiments where the backplate comprises PTFE or like material, a
portion of the PTFE material can comprise the electret portion 80
as is described below.
[0035] In various embodiments, electret portion 80 can comprise a
polymeric material having a high resistivity such as PTFE or
polycarbonate. The electret portion 80 has a permanent electrical
charge 81 which in turns confers an electrical charge 82 to the
backplate 50 with respect to diaphragm 60. This results in an
electric field strength 83 at backplate surface 51 depending upon
the strength of the charge and depth of the electret portion
beneath surface 51. As will be discussed herein, this field
strength can be preserved and protected through the use of a
hydrophobic coating. Charge 81 can be produced by bombarding the
electret material using e-beam or like electron bombardment methods
known in the art. Electret portion 80 can be positioned at various
depths and locations in the backplate. In embodiments shown in
FIGS. 6A and 6B, the electret portion is positioned below surface
51 of the backplate at a selectable depth depending on the desired
electric field strength. In another embodiment shown in FIG. 6C,
the electret portion is positioned at or proximate surface 51.
Also, in various embodiments, the electret portion 80 can be a
separate section that is embedded or attached to the backplate or
it can have unitary construction with the backplate. In the latter
case, the backplate can be electron bombarded as described herein
to produce the electret potion which comprise the same material as
the backplate, e.g., TEFLON. In other embodiments, the electret
portion comprises a layer or film 84 attached to surface 51.
Suitable films 84 for electret 80 can comprise various polymeric
fluorocarbon films such as PTFE, FEP, ETFE, CTFE and the like. In a
particular embodiment, the electret portion comprises a heat shrink
PTFE material such as Teflon.
[0036] Diaphragm 60 includes a first side 61 (also described as
surface 61) facing the backplate, a second side 62 (also described
as surface 62) facing away. The first side is offset from the
backplate by an offset distance 65 so as to allow the diaphragm to
vibrate back forth within the housing in response to sound waves
70. Offset distance 65 can be between 10 to 40.mu.m, more
preferably between 20 to 30.mu.m, with a specific embodiment of
25.mu.m. The offset can be defined by means of two or more spacers
66 which can be placed between the diaphragm and the backplate.
Spacers 66 typically comprise a non-conductive material such as
KAPTON or MYLAR and are preferably attached to the backplate by an
adhesive or joining means. Also the diaphragm can be supported
where it attaches to the housing interior surface 41 by means of
two supports 67. The diaphragm can be attached to the housing by an
adhesive or can be etched away from the housing itself using
photolithographic techniques known in the art.
[0037] Diaphragm 60 can comprise one or more thin flexible polymer
or metallic films known in the art. For the case of polymer films,
the diaphragm typically includes a conductive material such as a
conductive coating or laminate 63, with the conductive coating 63
on first side 61. In one embodiment, the diaphragm comprises a thin
metallic coated polyurethane or like material. In other
embodiments, the diaphragm can comprise thin PET films known in the
art such as MYLAR. The diaphragm will also typically be
electrically coupled to a wire 68 or other means of electrical
connection for output of electrical signals from the diaphragm to
another electrical component. Collectively, diaphragm 60 and
backplate 50 form a capacitor 90 that has a fixed charge 92, but a
variable capacitance 91 and variable voltage 93 (with respect to
the backplate and the diaphragm). Vibration of the diaphragm in
response to sound wave 70 results in an electrical interaction
between the diaphragm and the backplate so as to change capacitance
92 and voltage 93. Varying voltage 93 comprises the output signal
100 of the microphone. In many embodiments, the diaphragm is
electrically coupled to an integrated circuit (e.g., a chip) or
other electric device 110 which performs one or more functions on
signal 100 (e.g., pre-amplification) so as to produce a processed
signal 101
[0038] In many embodiments, the backplate and the diaphragm can be
coated with a hydrophobic coating 120 configured to prevent or
reduce wetting of the surfaces of these parts by water and aqueous
solutions (e.g., soap, pool water, etc). Further the hydrophobic
coating is desirably configured to prevent the condensation of
liquid water on these surfaces. This can be achieved by configuring
the coating to be sufficiently smooth and have a sufficiently low
surface energy such that water droplets can not wet and spread
across the backplate and/or diaphragm surfaces. In particular
embodiments, the coating is configured to prevent or minimize water
droplets from wetting the facing surfaces 51 and 61 of the
backplate and the diaphragm. Reduced wetting of the backplate can
be achieved in several different ways. Coating of the backplate
surface prevents condensation and wetting of the backplate
directly. While coating of the diaphragm reduces the formation of
liquid droplets on the diaphragm from condensation which may then
spread to the backplate. It also reduces the tendency of liquid
water from wicking in between the offset space between the
backplate and the diaphragm. Coating of both the backplate and the
diaphragm can achieve both of theses outcomes.
[0039] In addition to reducing condensation and wetting of the
backplate, the coating is desirably configured to prevent or
minimize an amount of condensation or other wetting of by liquid
water tending to neutralize the electric field strength 83 at the
backplate surface 51. In this respect, the coating serves to
preserve and protect field strength 83 and the microphone function
associated with the field strength (e.g., sensitivity, etc.). The
coating can be configured to do so even in high humidity high
temperature conditions such as that found in the ear canal (e.g.,
90% RH and 98.6.degree. F.) and when portions of the underlying
surfaces are at or below the dew point temperature such as might
occur when a hearing aid wearer goes from cool to warmer conditions
(e.g., from outside to indoors). In particular embodiments, the
coating can be configured to prevent condensation when ambient air
temperatures change by 10-20.degree. C. or more. Such
configurations serve to preserve the function of the microphone
including various microphone functional parameters such as the
sensitivity, bandwidth and signal to noise ratio. For example, the
coating can be configured to prevent no more than about a 2 dB loss
in the output signal from the microphone, when the microphone is
exposed to high humidity or varying ambient temperature conditions.
In use, the coating thus serves to preserve the function of the
microphone in various environmental conditions including high
humidity, high temperature conditions and cases where environmental
conditions rapidly change.
[0040] Coating 120 can be configured to perform a number of
functions. As discussed above, coating 120 serves to preserve the
field strength 83 in the presence of high humidity and changing
ambient conditions. The coating also serves to protect the long
term integrity of charge 81 of the electret portion by sealing the
electret portion from contact with liquid water and various
contaminants including conductive contaminants (e.g., cerumen,
soap, shampoo, conditioner, salt, chorine solutions and the like)
which may physically degrade the electret portion 80 or otherwise
cause dissipation of charge 81. Such protection can be useful for
example, when a hearing aid wearer having an embodiment of
microphone assembly passes near an external electromagnetic field
(e.g., a metal detector) which could induce leakage currents from
the electret portion to an adherent conductive contaminant.
[0041] The coating can be applied throughout the interior portions
of the microphone assembly. In various embodiments, the coating can
applied to one of more of backplate surfaces 51 and 52, diaphragm
surfaces 61 and 62 and to all or a portion of the microphone
assembly interior surface 41 as well as exterior surface 42. In
particular embodiments, the coating can be applied in around sound
port 45 and other points of fluid entry into the housing to prevent
liquid water from entering into the microphone housing via
capillary attraction. In one embodiment shown in FIG. 7A the
coating can be applied to the entire backplate and diaphragm
surface 61. This configuration serves to prevent the possibility of
water wicking in between the surfaces of the backplate and the
diaphragm, by preventing water from wetting either of the two
facing surfaces. In another embodiment shown in FIG. 7B, the
backplate and both sides of the diaphragm can be coated. In still
another embodiment shown in FIG. 7C, only the backplate is coated.
In these and other embodiments, all or a portion of the microphone
interior and/or exterior surfaces can be coated. Coating of the
exterior reduces the likelihood of moisture wetting the outside of
the housing, while coating of the interior particularly around
sound port 45 reduces the likelihood of water being able to wick
into the housing interior. In this way, the coating provides a dual
mode means of moisture protection by 1) by reducing the moisture
burden and thus hydrostatic pressure on the exterior of the
housing; and 2) reducing the likelihood of any liquid from actually
entering the housing. Further in particular embodiments, the
surface tension and thickness of the coating can be matched to the
diameter or other dimension of the sound port to enhance water
repelling properties at this location. For example, the coating at
housing can have a lower surface tension than that in other
locations can be used. Also the diameter or other major dimension
of the sound port can be made smaller to reduce or impede the entry
of liquid water by making it energetically unfavorable to do
so.
[0042] For embodiments where the diaphragm is coated, the coating
is desirably configured to have minimal effects on the acoustical
vibrations of the diaphragm as well as the electrical interactions
of the diaphragm with the backplate. For hearing aid and other
related applications, this allows the coated membrane to be
acoustically operable through the range of audible sounds such that
the hearing aid can provide an acoustical output to the wearer that
is a discernable representation of an audible sound inputted to the
diaphragm (meaning the user can recognize and understand the sound
e.g., words in a conversation, etc). This can be achieved through
control of the thickness and material properties of the coating
(e.g., density, stiffness, etc). Thin flexible coatings are
desirable in this regard. The thickness and stiffness of the
coating can be matched or otherwise selected depending upon those
or related properties of the diaphragm. In various embodiments, the
thickness of the coating can be in the range from about 0.5 to 5 um
and in preferred embodiments, is about 1 um. In specific
embodiments, the coating thickness and properties can be configured
to have a minimal effect on one or more of the stiffness, dampening
coefficient and resonant frequencies of the diaphragm. These values
can be measured using one or more test methods known in the art
such as ASTM test methods. Vibrational characteristics of the
diaphragm (e.g. resonant frequency) can be measured using a laser
Doppler vibrometer. The vibrational characteristics of the
diaphragm before and after coating can also be modeled using a
Bessel function. This or a similar function can be used to predict
the effects of a particular coating on the acoustical vibrations
and other properties of the diaphragm. Specifically, data can be
collected to make comparisons to Bessel parameters before and after
coating with a particular coating having a particular property set
(e. g, thickness and stiffness) and used to extrapolate to coatings
having different property sets. In various embodiments, the coated
diaphragm can be configured to have Bessel parameters that differ
by no more than a selected amount (e.g. <10%, <5%, <1%,
<0.5%, <0.1%, etc.) from an uncoated membrane.
[0043] In various embodiments, the coating can comprise one or more
hydrophobic polymers known in the art such as polyurethane and
polysiloxane. In preferred embodiments, the coating comprises a
fluoropolymer. Desirably the coating is substantially free of
pigments or other solids that can absorb water as well as any
solids that can cause surface asperities. Also desirably, the
coating has a low viscosity and surface energy allowing it to
readily spread/wet over and uniformly coat a low surface energy
material (such as PTFE) when applied by dip coating, spray coating
or like method. For dip coating, preferably the microphone housing
is dipped in the coating solution in a perpendicular orientation
with respect to the surface of the coating solution (i.e., the
sound port is perpendicular to the surface of the solution) so that
an assembly technician can see the air bubbles existing the housing
and know that solution is entering the interior of the housing. The
technician can then use the number of air bubbles as a gauge to
determine and control how much coating is entering the housing. The
number of air bubble can be titrated to the particular sized
housing where the performance of the microphone in humid
environments is tested after coating to determine the number of
bubbles. For example, two air bubbles can used for a housing having
dimensions of about 5 mm by 5 mm by 1 mm. In various embodiments,
the coating can have surface energy of between 11 to 15 dynes and
more preferably between 11 to 12 dynes/cm. Also desirably the
coating has a low temperature curing profile (e.g., room
temperature) and cures in a fast uniform manner so as to provide a
smooth surface with a uniform thickness. The former property
minimizes any possible thermal damage to various microphone
components including the backplate (including the electret
portion), diaphragm and the associated adhesives. In a preferred
embodiment, the coating comprises a fluorochemical acrylate
polymer, an example of which includes NOVEC Electronic Coating
EGC-1700, available from the Specialty Materials Division of the 3M
Corporation (St. Paul, Minn.). This coating has a surface energy of
between 11-12 dynes/cm (when dry) and can cure at room temperature.
The polymer comprising the coating can be readily diluted in a
hydrofluorether or like solvent to dry quickly to produce a clear
smooth uniform surface. This coating can be sprayed, dip coated or
brushed on. Also it has a volume resistivity of about
4.6.times.10.sup.12 ohm cm. Because of its low surface tension, the
coating solution can be readily applied and spread over an
underlying low surface energy substrate comprising the electret
portion of the backplate. Also, embodiments of the coating not only
provide protection against wetting by various aqueous solutions,
they also provide protection to coated structures against skin
oils, cerumen, dust and dirt since they present a low surface
tension inert coating which resists adhesion by particles and
solutions.
[0044] In various embodiments, of coating application methods, the
coating can be applied to one or more components of microphone
assembly 40 by spraying, dip coating, brushing and like methods. In
preferred embodiments, the coating is applied by dip coating. Dip
coating can be done by immersing the assembly for a period between
2 to 30 seconds and more preferably between 2 and 10 seconds. As
discussed above, the viscosity of the coating solution is desirably
configured to allow the coating to readily wet and spread across
the intended component for the particular application method. In
one embodiment, the entire microphone assembly is dipped in the
coating to allow the coating to wet the entire interior surface of
the housing including the backplate and diaphragm. In this
embodiment, the viscosity is configured to allow the coating to wet
the entire microphone interior via entry through the sound port. In
various embodiments, the viscosity of the coating solution can be
in the range of 1 to 5 centipoise and more preferably between to 2
to 3 centipoise. The housing interior can also be coated before it
is assembled, e.g., by individually coating portions of the
assembly that are later joined together. In other embodiments, the
diaphragm, backplate or other component can be individually dip
coated to allow coating of selected components only. In related
embodiments, portions of particular components can be masked off
using methods known in the art to allow coating of selected
portions of a particular component only, e.g., one side of the
diaphragm or backplate. These components can then be assembled into
the microphone assembly. Masking techniques can also be used to
allow selected coating of portions of the microphone assembly when
the microphone housing is in various stages of assembly.
[0045] The coating can be configured to be cured through a range of
temperatures (e.g., 20to 50.degree. C.). In many embodiments, the
coating is configured to be cured at or near room temperature
(e.g., about 22-27.degree. C.) to minimize the thermal effects on
various components of the assembly, e.g., diaphragm, backplate,
integrated circuits. This approach reduces the likelihood of
failure of these components from curing at higher temperatures. The
coating can also be configured to be cured by use of UV curing and
like methods. The coating is desirably configured to dry uniformly
so as to produce a uniform thickness and smooth surface with
minimal asperities. In various embodiments, this can be achieved by
the use fluoropolymer coating described herein such as the Novec
coating.
[0046] Various embodiments of the coating can be used for improving
the moisture resistance of microphones used in a number of
electro-acoustical applications. In many embodiments, the coating
is configured to be used for coating one or more microphone
components (e.g., the backplate, diaphragm, etc.) used in hearing
aids including CIC hearing aids. Such hearing CIC hearing aids can
include those configured to be positioned in the bony portion of
the ear canal for extended periods of wear, for example for a
periods of several weeks to six months or longer. Accordingly in
such embodiments, the coating is configured to provide condensation
and moisture protection to a hearing aid microphone assembly in the
warm moist thermal environment of the ear canal including that of
the bony portion (e.g., 90% RH and 98.6.degree. F.). Further, the
coating can be configured to provide such protection for periods of
extended wear from weeks to six months or longer. Also, the coating
can be configured to provide such protection when the wearer
rapidly changes his ambient environment by 10-20.degree. C. or more
such as when going from the cool outdoors to a heated indoor
environment. Suitable coatings for such applications include
fluorochemical acrylate polymers such as the 3M Novec coating
described herein.
[0047] Using one or more embodiments of the coatings and
application methods described herein, electret microphones can be
provided for hearing aid and other applications that have improved
moisture resistance and charge stability in adverse environmental
conditions tending to cause liquid condensation on internal
components of the microphone. The low surface energy of the coating
provides a moisture resistant microphone due to the fact that the
internal surfaces of microphone, such as those on the backplate,
are wet only with difficulty by liquid water that does not spread
but remains in a high contact angle configuration. Further, any
moisture that does happens to condense on the backplate cannot
readily form a continuous film and is thus impeded from wicking
into the working gap between the charged electret and the diaphragm
by a capillary action effect. In this respect, the coating
functions as a fluidic resister by impeding the hydrostatic forces
tending to drive capillary action across an uncoated surface
bounded by another uncoated surface such as those between backplate
and the diaphragm. Accordingly, the particular properties of the
coating such as surface tension can be titrated to provide a
desired amount of fluidic resistance to the hydrostatic driving
forces of a particular structure.
[0048] In other embodiments not shown, the moisture resistance of
microphone assemblies, such as those used in a CIC hearing aid, can
be further enhanced through the use of a fluidic barrier structure
positioned at or near sound port 45 or other portion of the
microphone housing. Such fluidic barrier structures are described
in further detail in U.S. patent application Ser. No. 60/696,265
which is fully incorporated herein by reference. As described in
the application, these barriers can be configured to prevent or
impede the ingress of liquid water or liquid into the microphone
housing.
CONCLUSION
[0049] The foregoing description of various embodiments of the
invention has been presented for purposes of illustration and
description. It is not intended to limit the invention to the
precise forms disclosed. Many modifications, variations and
refinements will be apparent to practitioners skilled in the art.
Further, the teachings of the invention have broad application in
the hearing aid fields, the microphone fields as well as other
fields which will be recognized by practitioners skilled in the
art. For example, the coating can be configured to be applied to a
number of different microphones in various fields including
condenser-based microphones used in recording, broadcasting and
amplification.
[0050] Elements, characteristics, or acts from one embodiment can
be readily recombined or substituted with one or more elements,
characteristics or acts from other embodiments to form numerous
additional embodiments within the scope of the invention. Hence,
the scope of the present invention is not limited to the specifics
of the exemplary embodiment, but is instead limited solely by the
appended claims.
* * * * *